This application is related commonly owned patent application Ser. No. 09/696,528, filed Oct. 23, 2000, entitled xe2x80x9cElectrostrictive Graft Elastomers.xe2x80x9d
This invention was jointly made by employees of the U.S. Government and an employee of the National Research Council and may be manufactured and used by or for the government for governmental purposes without the payment of royalties thereon or therefor.
1. Field of the Invention
This invention relates generally to actuators. It relates in particular to a device for providing an electromechanical response which has two polymeric webs bonded to each other along their lengths.
2. Description of the Related Art
Electromechanical actuators of the related art include ceramic actuators of various configurations, including those which are direct piezoelectric; polymer actuators, including direct piezoelectric, electrostatic, conductive polymer based, gel based, and ionomeric; and actuators of ceramic-polymer composites, including those which are direct piezoelectric. All of these actuators are found wanting, and, as a result their applicability is limited. For example, the ceramic actuators are generally heavy and brittle and they provide a small displacement/strain. Polymer actuators generally also provide either a small displacement/strain (e.g., PVDF), they are subject to environmental limitations (e.g., ionomer), or they have low output force due to low mechanical modulus (e.g., polyurethane, silocone elastomer). For example, ionomeric polymers generally require a humid environment for operation. Ceramic-polymer composite actuators also exhibit a small displacement/strain, and they are generally heavy and brittle. Examples of polymer-polymer bi-layer configurations of the related art are found in U.S. Pat. No. 4,400,634 and U.S. Pat No. 4,330,730, the former being based on Maxwell Stress (i.e., electrostatics), and the latter being based on the piezoelectric effect. Neither of these patents discloses or suggests employing a layer which exhibits electrostriction, viz., by rotation of polar graft moieties within the layer, nor is there any comprehension or suggestion therein of the advantageous results achieved by providing at least one layer which exhibits electrostriction.
It is the primary object of the present invention to obviate the inadequacies of related art devices and provide an actuator which has a low mass and a unified body, is simple to operate, has a long life, is tough, and is easily adjustable and completely functional in all environments.
This object, and other attending benefits, was achieved by the provision of a device which includes two polymeric webs bonded to each other along their lengths. At least one polymeric web is activated upon application thereto of an electric field and exhibits electrostriction by rotation of polar graft moieties within the polymeric web.
In one preferred embodiment, one of the two polymeric webs is an active web upon application thereto of the electric field, and the other polymeric web is a non-active web upon application thereto of the electric field. In this embodiment the active web is composed of an electrostrictive graft elastomer and the non-active web is composed of any other flexible polymer. Such an electrostrictive graft elastomer is described and claimed in xe2x80x9cElectrostrictive Graft Elastomersxe2x80x9d, Ser. No. 09/696,528, filed Oct. 23, 2000, hereby incorporated by reference. The electrostrictive graft elastomer has a backbone molecule which is a non-crystallizable, flexible macromolecular chain, preferably a member selected from the group consisting of silicones, polyurethanes, polysulfides, nitrile rubbers, polybutenes, and fluorinated elastomers, e.g., a chlorotrifluoroethylene-vinylidene fluoride copolymer. The electrostrictive graft elastomer contains a grafted polymer which is a homopolymer or a copolymer which forms polar crystal phases and physical entanglement sites with backbone molecules, preferably a member selected from the group consisting of poly(vinylidene fluoride), poly(vinylidene fluoride-trifluoroethylene) copolymers, poly(vinylidene fluoride-trifluoroethylene) copolymers, poly(trifluoroethylene), vinylidene-trifluoroethylene copolymers, ferroelectric nylons (odd-numbered nylons), cyanopolymers (polyacrylonitriles, poly(vinylidene cyanide, vinylidene cyanide-based copolomeers, poly(cyanoaryl ether)), polyureas, polythioureas, ferroelectric liquid crystal polymers and piezoelectric bipolymers. In this embodiment the non-active web is composed of any other flexible polymer, preferably a member selected from the group consisting of silicones, polyurethanes, polysulfides, nitrile rubbers, polybutenes, and fluorinated thermoplastic elastomers, e.g., a chlorotrifluoroethylene-vinylidene fluoride copolymer.
In another preferred embodiment, both of the two polymeric webs are capable of being active webs upon application thereto of the electric field, the two polymeric webs being alternately activated and non-activated by the electric field. In this embodiment, both of the polymeric webs are composed of an electrostrictive graft elastomer having a backbone molecule which is a non-crystallizable, flexible macromolecular chain, preferably a member selected from the group consisting of silicones, polyurethanes, polysulfides, nitrile rubbers, polybutenes, and fluorinated elastomers, such as a chlorotrifluoroethylene-vinylidene fluoride copolymer. The electrostrictive graft elastomer contains a grafted polymer which is a homopolymer or a copolymer which forms polar crystal phases and physical entanglement sites with backbone molecules, preferably a member selected from the group consisting of poly(vinylidene fluoride), poly(vinylidene fluoride-trifluoroethylene) copolymers, poly(vinylidene fluoride-trifluoroethylene) copolymers, poly(trifluoroethylene), vinylidene-trifluoroethylene copolymers, ferroelectric nylons (odd-numbered nylons), cyanopolymers (polyacrylonitriles, poly(vinylidene cyanide, vinylidene cyanide-based copolomeers, poly(cyanoaryl ether)), polyureas, polythioureas, ferroelectric liquid crystal polymers and piezoelectric bipolymers.
In both preferred embodiments, the two polymeric webs are bonded to each other by bonding means selected from the group consisting of chemical bonding, physical bonding, mechanical bonding, and biological bonding. Preferred is a chemical bonding means employing a chemical adhesive, which is cast and cured at ambient temperature to form an integrating layer of less than 1 micrometer thickness positioned between the two polymeric webs.
The resulting actuator achieves the primary object set forth above. Moreover, it provides a large displacement, high energy density, and a double frequency response as a result of a quadratic relationship of strain with applied electric field. Such a double frequency response is not evident in ionomer, piezoelectric or conducting polymer actuators. Furthermore, the actuator of the present invention does not require the humid environment which is necessary for ionomer actuators. Applications for electromechanical actuators according to the present invention include: motion control, position control, tension control, curvature control, biomedical pulse control, surface flow dynamic control, display panels, optical alignment, optical filters, micro-electromechanical systems, and nano-electromechanical systems.